<!--
!!html_title Lines demo 2 - Computer Graphics from scratch
-->
<canvas id="canvas" width=600 height=600 style="border: 1px grey solid">

  <script>

    // ======================================================================
    //  Low-level canvas access. 
    // ======================================================================

    var canvas = document.getElementById("canvas");
    var canvas_context = canvas.getContext("2d");
    var canvas_buffer = canvas_context.getImageData(0, 0, canvas.width, canvas.height);
    var canvas_pitch = canvas_buffer.width * 4;


    // The PutPixel() function.
    var PutPixel = function (x, y, color) {
      x = canvas.width / 2 + Math.floor(x);
      y = canvas.height / 2 - Math.floor(y) - 1;

      if (x < 0 || x >= canvas.width || y < 0 || y >= canvas.height) {
        return;
      }

      var offset = 4 * x + canvas_pitch * y;
      canvas_buffer.data[offset++] = color[0];
      canvas_buffer.data[offset++] = color[1];
      canvas_buffer.data[offset++] = color[2];
      canvas_buffer.data[offset++] = 255; // Alpha = 255 (full opacity)
    }


    // Displays the contents of the offscreen buffer into the canvas.
    var UpdateCanvas = function () {
      canvas_context.putImageData(canvas_buffer, 0, 0);
    }


    // ======================================================================
    //  Rasterization code.
    // ======================================================================

    // A Point.
    var Pt = function (x, y) {
      if (!(this instanceof Pt)) { return new Pt(x, y); }

      this.x = x;
      this.y = y;
    }


    var Interpolate = function (i0, d0, i1, d1) {
      if (i0 == i1) {
        return [d0];
      }

      var values = [];
      var a = (d1 - d0) / (i1 - i0);
      var d = d0;
      for (var i = i0; i <= i1; i++) {
        values.push(d);
        d += a;
      }

      return values;
    }


    var DrawLine = function (p0, p1, color) {
      var dx = p1.x - p0.x, dy = p1.y - p0.y;

      if (Math.abs(dx) > Math.abs(dy)) {
        // The line is horizontal-ish. Make sure it's left to right.
        if (dx < 0) { var swap = p0; p0 = p1; p1 = swap; }

        // Compute the Y values and draw.
        var ys = Interpolate(p0.x, p0.y, p1.x, p1.y);
        // console.log(ys);
        for (var x = p0.x; x <= p1.x; x++) {
          console.log("-------", x, ys[(x - p0.x) | 0]);
          PutPixel(x, ys[(x - p0.x) | 0], color);
        }
      } else {
        // The line is verical-ish. Make sure it's bottom to top.
        if (dy < 0) { var swap = p0; p0 = p1; p1 = swap; }
    
        // Compute the X values and draw.
        var xs = Interpolate(p0.y, p0.x, p1.y, p1.x);
        
        for (var y = p0.y; y <= p1.y; y++) {
          console.log("-------", xs[(y - p0.y) | 0], y);
          PutPixel(xs[(y - p0.y) | 0], y, color);
        }
      }
    }

    // DrawLine(Pt(-200, -100), Pt(240, 120), [0, 0, 0]);
    DrawLine(Pt(-50, -200), Pt(60, 240), [0, 0, 0]);

    UpdateCanvas();

  </script>